The Windows 8 operating system is in its final stages of development and may ship toward the end of this year. But Microsoft doesn't want consumers to hold off on purchasing Windows 7 PCs in the meantime.

That's where the comes in. Announced on Thursday along with the latest test version of Windows 8, the Windows Upgrade Offer will let buyers of Windows 7 PCs upgrade to Windows 8 Pro for US$14.99.

Microsoft hasn't disclosed retail prices for shrink-wrapped, stand-alone versions of Windows 8, but the equivalent to Windows 8 Pro is Windows 7 Professional, which is $199.99.

To qualify for the discount, consumers must buy a new Windows 7 PC between Saturday, June 2 and Jan. 31 of next year. They will have until Feb. 28, 2013, to download the Windows 8 Pro upgrade. The offer is available in 131 markets.

The maximum number of new Windows 7 PCs a consumer can buy as part of the program is five. Windows 7 editions that qualify for the offer are Home Basic, Home Premium, Professional and Ultimate. That leaves out of this offer's scope Windows 7 Starter, which is the most basic for consumers, and Windows 7 Enterprise, which is designed for workplace PCs.